#include <stdio.h>
#include <math.h>

int main(void) {
  double data[3];
  scanf("%lf%lf%lf", &data[0], &data[1], &data[2]);
  for (int k = 0; k < 3; k++) {
    for (int j = 0; j < 2; j++) {
      if (data[j] > data[j + 1]) {
        double tmp = data[j];
        data[j] = data[j + 1];
        data[j + 1] = tmp;
      }
    }
  }
  
  if (data[0] + data[1] < data[2]) {
    printf("Not a triangle.");
    return 0;
  }
  
  double d = (data[0] + data[1] + data[2]) / 2.0;
  printf("%.2f", sqrt(d * (d - data[0]) * (d - data[1]) * (d - data[2])));
  return 0;
}
